Stable Joints for Concrete Floors: Differential movement versus load-transfer efficiency

Pavement engineers, who deal with many of the same issues faced in the concrete-floor field, do not talk much about joint stability or differential movement. Instead, they talk about load-transfer efficiency—a related but distinct property.

Whenever a load is applied to one side of a joint, it creates stress on the loaded side. Load transfer occurs when some of that stress gets transferred to the unloaded side. Load transfer efficiency (LTE) is a measure of how well the joint shifts stress to the unloaded side.

A load is applied to one side of the joint and the deflection on that side is measured. At the same time, the deflection on the other, unloaded side, is measured. The reading on the unloaded side is divided by the reading on the loaded side, then the answer is converted to a percentage—the LTE. The best possible result is when both sides deflect by the same amount, meaning the LTE is reported as 100 per cent. If the unloaded side does not move at all, that is the worst possible result, with an LTE of zero. Pavement engineers generally like to see LTEs above 75 per cent. (Strictly speaking, the procedure described generates what is known as deflection-based LTE. Some highway designs rely instead on stress-based LTE, which is normally lower. Stress-based LTE can be measured, but is usually inferred from the more easily tested deflection-based LTE.)

How does this method differ from the differential movement discussed in ACI 360?
Differential movement is a reading of actual slab deflection, in millimetres or thousandths of an inch. LTE, in contrast, is the ratio of two deflection readings. The actual deflections that determine LTE can be large or small. Consider a test for LTE in which the loaded side deflects by 0.10 mm (0.004 in.) and the unloaded side deflects by 0.04 mm (0.002 in.). This gives an LTE of 40 per cent, which is poor by pavement standards. However, the actual deflections are tiny, and the arithmetic difference between them is just 0.06 mm (0.002 in.). Movement like that would have almost no effect on floor usage. In a test in which the loaded side deflects by 2.5 mm (0.098 in.), and the unloaded side by 2 mm (0.079 in.), the LTE would be 80 per cent, which many pavement engineers would find acceptable. However, now the differential deflection is 0.50 mm (0.019 in.)—a reading almost certain to cause trouble in a warehouse or factory floor.

It makes sense pavement engineers focus more on load transfer than on differential movement. The vehicles that travel on pavements—cars, trucks, and airplanes—have springs and big, pneumatic tires. Such vehicles are not affected much by differential movement. Pavement engineers worry about load transfer not because the vehicles they design for require stable joints, but rather because load transfer reduces the maximum stress within the concrete slabs. With lower stress, pavements last longer and can sometimes be made thinner and cheaper.

In contrast, many of the vehicles that travel on concrete floors have stiff suspension systems and small, hard tires. Such vehicles are hard on joints, and the problems get worse as differential movement increases. This is why, in most cases, differential movement is better than LTE at predicting concrete floor serviceability.

This is not to say LTE is irrelevant in floors. Structural engineers pay attention to it when they determine floor-slab thickness. Further, anything that improves load transfer will also reduce differential movement, which is all positive. However, if design professionals want to know whether a floor joint will hold up under traffic, differential movement is the property to look at.

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